1. Field of the Invention
The present invention relates generally to a received signal processing technique in a CDMA mobile terminal, and in particular, to an apparatus for reducing intermodulation distortion generated while processing a received signal.
2. Description of the Related Art
An existing digital CDMA (Code Division Multiple Access) mobile terminal has a transmission frequency band of 824-849 MHz and a reception frequency band of 869-894 MHz. An analog AMPS (Advanced Mobile Phone System) mobile terminal also has the same transmission and reception frequency bands. Since the CDMA mobile terminal and the AMPS mobile terminal have the same frequency bands, the CDMA mobile terminal is so designed to reduce and/or eliminate interference with the AMPS signals. That is, the CDMA mobile terminal is so designed as to have the constant reception performance, even though there exist AMPS signals around the CDMA channel.
In this connection, there has been proposed an IS-95 intermodulation distortion characteristic specification. The CDMA mobile terminal should be so manufactured as to satisfy this specification. For reference, the IS-95 intermodulation distortion characteristic specification is shown in Table 1 below.
TABLE 11st Jammer Signal2nd Jammer SignalTraffic Signal(±900 KHz Offset)(±1700 KHz Offset)1−101 dBm−43 dBm−43 dBm2 −90 dBm−32 dBm−32 dBm3 −79 dBm−21 dBm−21 dBm
In the prior art, there have been proposed several methods for satisfying the above conditions. However, the proposed methods are all performed by software. That is, in the existing methods, the mobile terminal operates according to software processing to satisfy the above-stated specification.
FIG. 1 shows a typical structure of the conventional CDMA mobile terminal. Specifically, FIG. 1 shows a structure of a signal processor for processing transmission and reception signals.
Referring to FIG. 1, a receiver of the CDMA mobile terminal includes a duplexer 100, a low-noise amplifier (LNA) 112, an RF (Radio Frequency) filter 114, a mixer 116, and an IF (Intermediate Frequency) filter 118. Further, the receiver includes an attenuator 110 provided in front of the low-noise amplifier 112 to satisfy the intermodulation distortion (IMD) characteristics. The attenuator 110 is provided to solve a reception performance degradation problem due to the intermodulation noise components which are generated when the two CDMA and AMPS continuous waves having the same frequency band are input to the low-noise amplifier 112. The attenuator 110 solves the reception performance degradation problem by properly controlling a gain of the signals applied to the low-noise amplifier 112. The IF filter 123 inputs the modulated IF signals and filters the predetermined transmitting IF frequency signals. The mixer 121 mixes the output of the IF filter 123 with the predetermined local oscillating signals and thus outputs the RF signals. The RF filter 119 filters the mixed signals and detects the RF signals. The drive amplifier 117 amplifies the level of the RF signals so as to obtain the power for driving the power amplifier 113 as stated below. The duplexer 100 switches the connection of the antennas and the transmitting and receiving portion. The RF filter 115 filters the output of the driving amplifier 117, once again. The power amplifier 113 amplifies the filtered RF signals to the predetermined level. The isolator 111 is connected between the output end of the power amplifier 113 and the input end of the duplexer 100 so as to avoid the transmitting distortion of the reflection wave, thereby transmitting the output of the power amplifier 113 to the duplexer 100 without any attenuating. The duplexer 100 switches antenna 130 between a transmitting and receiving mode.
Now, an operation of the CDMA mobile terminal will be described in detail. In accordance with the unique characteristics of the CDMA system, a level of an IF signal output from the mixer 116 has a constant value regardless of the strength of a signal received at an antenna ANT. The mixer 116 performs this operation under the control of a controller (now shown).
For information, a voltage level for controlling the mixer 116 is adjusted depending on the strength of the signals received at the receiver. Such a power control method is generally called “open-loop power control”. There is a case where the received signal strength becomes higher than a predetermined level. In this case, the controller attenuates the received signal depending on the received signal strength by outputting an intermodulation distortion (IMD) characteristic control signal. To this end, the controller generates a control signal for properly controlling a gain of the attenuator 110. This control signal is generally referred to as an “IMD characteristic control signal” or “IMD control voltage”. By doing so, it is possible to decrease the intermodulation distortion, thereby solving the reception performance degradation problem.
In this case, however, the conventional CDMA mobile terminal of FIG. 1 has the following disadvantages. That is, the attenuator 110 is controlled through a software process which is performed by the controller. Hence, this method inevitably includes an additional software process, making it difficult to control the attenuator 110 in real time. Further, the additional software process may raise an overload problem of the controller. In particular, since the attenuator 110 is controlled by software, it is very difficult to match the correct control timing.
In other words, the controller calculates the control signal for the attenuator 110 by performing the software process on the received jammer signals, so that the control signal will be generated after certain time delay. In addition, since the hardware interacts with the software, it is necessary to control an accurate operating point.